LOW LATITUDE CONTROL OF MARINE PRODUCTIVITY OFF THE WESTERN MARGIN OF NORTH AMERICAN DURING THE LAST 52 KA?

The influential study of Behl and Kennett (1996) on the laminations of the Santa Barbara Basin (SBB) off Southern California has linked shifts in bottom water oxygen content to the millennial-scale, interstadial climate events recorded in the GISP-2 and GRIP oxygen isotopic records from Greenland ice. Here we demonstrate clear correlations between these high-resolution polar ice core records, and records of sediment diffuse spectral reflectance, organic carbon, and calcium carbonate concentration, in cores raised from the Magdalena margin off Baja California, Mexico. The Magdalena margin site exhibits a remarkably linear sedimentation rates of ~30 cm/ka over the past 52 ka and is located ~1500 km south of the Santa Barbara Basin. The distal location of the Baja site relative to the ventilation zones of the Northwest Pacific and its proximity to the equatorial Pacific, calls into question the relative importance of high latitude ventilation versus low-latitude productivity changes as the dominant mechanism controlling sedimentary organic carbon content along the eastern margin of North America. We evaluate the relative importance of these two mechanisms using multiple proxies of marine productivity, and foraminiferal fragmentation as a proxy for carbonate dissolution. At the Magdalena margin site off Baja California, peaks in organic carbon content which occur during warm interstadial events correspond to maxima in benthic foraminiferal absolute abundance that are as much as two order of magnitude greater than those observed during cool stadial events. The maxima in benthic foraminiferal accumulation likely represent minimal estimates of the true accumulation as they occur at time of slightly greater carbonate dissolution as measured by the absolute concentration of foraminiferal fragments. These results suggest a mechanism arising from changes in marine productivity, rather than the ventilation of the north Pacific as was originally proposed. One possible interpretation is that the deep nutricline and low productivity in the region associated with modern El Nino conditions was dominant during the cooler climate intervals of the past 52 ka.